1. Field of the Invention
This invention relates to anchoring systems for high tensile strength steel wire wrapped in tension around a core of a concrete pipe to prestress the pipe in radial compression, thereby enabling it to withstand substantial internal fluid pressure without failing.
2. Description of the Prior Art
U.S. Pat. No. 3,278,128 (issued Oct. 11, 1966 to Szulc) describes one type of prestressed concrete pipe made before this invention. U.S. Pat. No. 3,258,033 (issued June 28, 1966 to Ohnstad) discloses a prior art anchoring system used for prestressing wire in concrete pipe.
In the Szulc patent referred to above, a first prestressing wire was wrapped in tension around concrete pipe to form a helix with fairly wide spacing between adjacent turns. The first wire was secured at its opposite ends to embedment anchors cast in the concrete pipe to keep the pipe in radial compression. Thereafter, a second wire was wrapped in tension around the pipe between adjacent turns of the first wire, and the second wire was anchored at its opposite ends to keep additional compressive force on the pipe. Later, the practice was developed in which the first wire was wrapped around a tubular concrete pipe core with more closely spaced adjacent turns in the helix. The first wire was covered with a first layer of mortar, which was cured. Thereafter, the second wire was wrapped around the first layer of mortar in the form of a helix with adjacent turns also spaced fairly closely together. The second wire was coated with a second layer of mortar, which was cured. This procedure could be repeated as many times as necessary for the particular job involved, but, as explained below, two wrappings of wires were usually the practical maximum number.
It was the prior art practice to place the embedment anchors for the prestressing wires flush with the outside diameter of the pipe core. The embedment anchors had outwardly opening recesses which were temporarily filled with removable rubber plugs to keep the recesses from filling with concrete when the pipe core was cast. After the pipe core cured, the rubber plugs were removed from the two embedment anchors for the first prestressing wire, and anchor blocks were inserted in the recesses. The first wire was wrapped and coated with the first layer of mortar. After the mortar cured, it was manually cut or sculptured to form inclines in the first mortar coating down to the level of the embedment anchors in the pipe core for the second prestressing wire. The rubber plugs were removed and replaced with anchor blocks. The second wire and second mortar coating was then applied.
This procedure, although time-consuming and dependent on the craftmanship of the worker making the pipe, is satisfactory when only two layers of prestressing wire are required. However, when additional layers of prestressing wire are needed, the ramps or inclines cut in the mortar coatings become excessively long and difficult to make. The cuts are especially difficult to make at the top of the pipe, which often is 20 feet or more above ground. Moreover, the quality of the pipe depends on the ability of the workman to cut a smooth, gradual ramp that will not introduce wire lift-out forces at the anchor block, or cause undue stress concentrations in the wire. The wires were also electrically insulated from each other, making it difficult to apply cathodic protection, when needed.